1. Field of the Invention
The present invention relates generally to color video signal processing circuits, and more particularly, is directed to a circuit for obtaining individually a chrominance signal which is separated from a color video signal and a luminance signal which is separated also from the color video signal and modified to have a predetermined level-frequency characteristic.
2. Description of the Prior Art
In a recording circuit arrangement employed in a video tape recorder (VTR), luminance and chrominance signals are separated respectively from a color video signal which is supplied, for example, through a color television broadcast. The separated luminance signal is used for frequency-modulating an appropriate carrier to produce a frequency-modulated luminance signal. The separated chrominance signal is frequency-converted to have a frequency band lower than the frequency band of the frequency-modulated luminance signal. Then, the frequency-modulated luminance signal and the frequency-converted chrominance signal are mixed or combined to form a composite video signal which is supplied to a rotary magnetic head device to be recorded on a magnetic tape.
For each of such separations of the chrominance and luminance signals from the color video signal, a so-called comb-filter is ordinarily used. In the case where a color video signal according to the NTSC system is processed, a subtraction-type comb-filter which includes a delay device providing a signal delay of one horizontal period (hereinafter, referred to as a 1H delay device) is used for separating the chrominance signal from the color video signal. An addition-type comb-filter which also includes the 1H delay device is used for separating the luminance signal from the color video signal. To obtain the chrominance signal components contained in the color video signal, the subtraction-type comb-filter is formed to have a toothed band pass characteristic which provides a minimum gain for signal components residing in a frequency band lower than the frequency band of the chrominance signal in the color video signal, a depression falling into a minimum gain at each frequency represented by N.times.f.sub.H, where N is a positive integer and f.sub.H is a horizontal frequency, and a peak reaching to a gain of zero decibels (hereinafter, referred to as 0 dB) at each frequency represented by (N +1/2) X f.sub.H for signal components residing in the frequency band of the chrominance signal components in the color video signal.
To obtain the luminance signal contained in the color video signal, the addition-type comb-filter is formed to have another toothed band pass characteristic which provides a gain of 0 dB for the signal components residing in a frequency band lower than the frequency band of the chrominance signal in the color video signal, a peak reaching to a gain of 0 dB at each frequency represented by N.times.f.sub.H and a depression falling into a minimum gain at each frequency represented by (N +1/2) X f.sub.H for the signal components residing in a frequency band of the chrominance signal in the color video signal.
In a reproducing circuit arrangement of the video tape recorder, the frequency-modulated luminance signal and the frequency-converted chrominance signal are separated through a high pass filter and a low pass filter respectively from the composite video signal which is reproduced by the rotary magnetic head device from the magnetic tape. Then, a reproduced luminance signal is obtained by frequency-demodulating the frequency-modulated luminance signal separated from the reproduced composite video signal and a reproduced chrominance signal is obtained by frequency-converting the frequency-converted chrominance signal separated from the reproduced composite video signal to cause the same to reside in its original frequency band. The reproduced luminance and chrominance signals are combined with each other to be derived from an output terminal as a reproduced color video signal.
The reproduced color video signal thus obtained from the video tape recorder is supplied to, for example, a color video monitor having an image display device such as a color cathode ray tube. In the color video monitor, chrominance and luminance signals are separated from the reproduced color video signal respectively through subtraction-type and addition-type comb-filters having their respective toothed band pass characteristics similar to those described above. That is, in the event of a color video monitor for an NTSC system color video signal, a subtraction-type comb-filter which includes a 1H delay device and a toothed band pass characteristic which provides a depression falling into a minimum gain at each frequency represented by N.times.f.sub.H and a peak reaching to a gain of 0 dB at each frequency represented by (N+1/2) .times.f.sub.H for the reproduced color video signal, is used for separating the chrominance signal from the reproduced color video signal. An addition-type comb-filter which includes the 1H delay device and a toothed band pass characteristic which provides a peak reaching to a gain of 0 dB at each frequency represented by N.times.f.sub.H and a depression falling into a minimum gain at each frequency represented by (N+1/2).times.f.sub.H for the reproduced color video signal, is used for separating the luminance signal from the reproduced color video signal.
Assume the case where the luminance signal is obtained through the addition-type comb-filter provided in the recording circuit arrangement of a video tape recorder. The comb-filter has a toothed band pass characteristic which provides a constant gain of 0 dB for the signal components residing in a frequency band lower than the frequency band of the chrominance signal in the color video signal, as described above, and therefore has a constant level in a frequency band lower than the frequency band of the chrominance signal. When such a luminance signal is contained in the reproduced color video signal which is supplied to the color video monitor, there is the problem that the luminance signal separated from the reproduced color video signal through the addition-type comb-filter provided in the color video monitor has signal components of frequencies coinciding with each frequency represented by (N +1/2) .times.f.sub.H and their vicinities which are considerably attenuated compared with other signal components thereof. Such attenuated signal components of the luminance signal separated from the reproduced color video signal give rise to a deterioration in the quality of the reproduced picture which is obtained by the color video monitor based on the luminance and chrominance signals separated from the reproduced color video signal.
For compensating for the attenuation of the luminance signal brought about in the color video monitor as mentioned above, the luminance signal should preferably contain signal components which are of frequencies between each successive two of the frequencies represented by N.times.f.sub.H and which are emphasized in level within a predetermined frequency band. A level control circuit can be used for emphasizing the levels of selected signal components of the luminance signal separated from the color video signal in the recording circuit arrangement of the video tape recorder. However, when a level control circuit is provided in addition to the comb-filter and the level control circuit contains a lH delay device which is in addition to the 1H delay device contained in the comb-filter, the configuration of the recording circuit arrangement of the video tape recorder is made complicated and the production cost of the video tape recorder is undesirably increased.